Induction of DNA strand breaks and expression of HSP70 and GRP78 homolog by cadmium in the marine sponge Suberites domuncula

The marine sponge Suberites domuncula was used as a bioindicator to study t he effects of cadmium on the occurrence of DNA strand breakage and on the i nduction of the expression of the stress biomarkers, heat shock protein 70 (HSP70) and glucose-regulated protein 78 (GRP78) homolog. The cDNA encoding GRP78 homolog from S. domuncula was isolated and characterized. The GRP78 cDNA has a length of 2.1 kb and displays characteristic features of the HSP 70 family; it encodes an aa sequence of M-r 72,000. Exposure of S. domuncul a to 1 mg/L of cadmium chloride for 24 h caused a strong (16.6-fold) increa se in cadmium content to 7.7 mu g/g wet weight of sponge tissue; after an i ncubation period of 6 days, the accumulation was 20.4-fold. The increase in cadmium content was paralleled by a transient decrease in zinc content at days 1 and 3. Exposure of S. domuncula to cadmium chloride also resulted in a marked increase in the number of DNA single strand breaks, as assessed b y a recently developed fast and sensitive microplate assay. The maximum inc rease in DNA damage was observed after an incubation of 12 h in the presenc e of 1 mg/L of cadmium chloride; after longer incubation, the number of dam aged sites decreased, most likely due to DNA repair. Quantitative analysis of the expression of HSP70 (M-r 73 kDa) revealed that onset of maximal leve ls of HSP70 depends on the concentration of cadmium chloride in the ambient seawater. Maximal induction (8.9-fold increase compared to control) of HSP 70 following exposure to 1 mg/L of cadmium chloride was found after 12 h, w hile longer incubation periods (3-6 days) were needed to reach maximum leve ls of HSP70 in the presence of lower concentrations of cadmium chloride (0. 1 mg/L and 0.01 mg/L). Northern blot analysis revealed that the level of th e 2.0 kb sponge GRP78 homolog mRNA transiently increased under cadmium stre ss; the maximum increase in the presence of 0.1 mg/L of cadmium chloride wa s observed at day 3. Our results suggest that sponges are useful indicator organisms to assess the genotoxic risks of cadmium pollution in marine envi ronments.